Actuators are relatively simple mechanical components that are often incorporated into more complex mechanical systems, including those found in automobiles, airplanes, manufacturing facilities, and processing facilities. A conventional solenoid is one example of an actuator that has found broad application across many types of industries and technologies.
Shape memory alloys (SMAs) are metals that exist in two distinct solid phases, referred to as Martensite and Austenite. Martensite is relatively soft and easily deformed, whereas Austenite is relatively stronger and less easily deformed. SMAs can be induced to change phase by changes in temperature and changes in mechanical stress. Also, SMAs can generate relatively large forces (when resistance is encountered during their phase transformation) and can exhibit relatively large movements as they recover from large strains. SMAs have been used commercially in many types of actuators, where a temperature change is used to control the actuation cycle. One of the most widely recognizable applications has been the use of SMA based actuators in automatic sprinkler systems.
One disadvantage of SMA actuators triggered by changes in temperature is that a heating or cooling device must be incorporated into the actuator, increasing the size, expense, and complexity of the actuator. Further, the response of such an actuator depends on heat transfer, which can occur too slowly for certain applications. Material scientists have more recently recognized that the phase change between Martensite and Austenite can be induced by changes in an applied magnetic field in certain alloys, as well as by changes in temperature and stress loading. Because magnetic fields generated with electromagnets can be rapidly switched on and off, particularly compared to the time required to induce a change in temperature to initiate an actuation, electromagnetically controlled SMA based actuators appear to offer promise in applications where rapidly responding actuation is required. Such alloys are referred to as ferromagnetic shape memory alloys (FSMAs).
A spring-based FSMA actuator (as reported by T. Wada, and M. Taya. 2002. Proc. of SPIE on Smart Structures and Materials, ed. C. S. Lynch. 4699:294-302, the disclosure of which is hereby specifically incorporated herein by reference) has been designed and tested with favorable results. The specific FSMA employed was an alloy of iron and palladium (FePd), and the actuator described was triggered using a hybrid system including a permanent magnet and an electromagnet. The permanent magnet alone is insufficient to induce the phase change, but does enable a smaller electromagnet to be employed. Unfortunately, the cost of palladium is so prohibitive that commercial utilization of FePd based actuators is not now economically feasible.
In an attempt to identify other materials that could be of use in FSMA actuators, composites of a ferromagnetic material and a SMA alloy that itself is not ferromagnetic have been suggested (Y. Matsunaga, T. Tagawa, T. Wada, and M. Taya, et al. 2002. Proc. SPIE on Smart Materials, (March 17-21):4699:172, the disclosure of which is hereby specifically incorporated herein by reference). Matsunaga et al. describe a three layer composite in which a soft iron (Fe) core is sandwiched between two layers of a super elastic (but non ferromagnetic) SMA. The ferromagnetic material is iron, or an iron, cobalt, and vanadium alloy (FeCoV), and the SMA is an alloy of titanium and nickel (TiNi), or an alloy of titanium, nickel, and copper (TiNiCu). This approach enables a SMA material having good mechanical properties to be combined with a material having good magnetic properties to achieve a desirable FSMA composite. While such research indicated that FSMA composites are indeed achievable, the FSMA composites produced did not perform as well in actuators as did the FePd material.
It would therefore be desirable to produce FSMA composite having properties suited for use in actuators at a commercially viable cost. Because there exist many potential combinations of ferromagnetic materials and (non ferromagnetic) SMAs, it would further be desirable to provide a model to aid in identifying potentially useful components from which future composite ferromagnetic SMAs can be produced. Because the physical geometry of a FSMA component impacts the properties of that component, it would also be desirable to determine a specific geometry that provides enhanced performance when the material is utilized in an actuator. Finally, it would be desirable to develop different embodiments of actuators incorporating FSMAs that will likely have commercial value.